The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. A method and apparatus for determining at least one patient-related parameter for monitoring a patient, comprising using a general population-related non-linear pressure/CSA relationship of the arterial vascular bed and measuring the arterial pressure of the patient to obtain a prediction of the cross sectional area (CSA) of the thoracic part of the aorta, and to a method and apparatus for determining at least one patient-related parameter for monitoring a patient, comprising using a general population-related non-linear CSA/pressure relationship of the arterial vascular bed and measuring the CSA to obtain a prediction of the arterial pressure.
In U.S. Pat. No. 3,841,313 it has been suggested that the systolic area could be used to determine stroke volume. The systolic area is the area between the blood pressure and end diastolic pressure during systole. There have been published many modifications of this so-called pulse contour technique, see for instance U.S. Pat. No. 5,183,051. One of the major shortcomings of these pulse contour methods is their reliance upon measuring morphological features of the blood pressure waveform. In particular, the position of the upstroke and the position of the dicrotic notch, which positions indicate the opening and closing of the aortic valve respectively, must be found in order to measure the systolic area. In patients the dicrotic notch appears difficult to detect. WO 9724690 discloses an apparatus which determines the opening and closing time of the aortic valve by means of a three element windkessel model of the circulation. Said apparatus also functions in those cases where the heartbeat is irregular.
U.S. Pat. No. 5,400,793 describes another method for determining the stroke volume from aortic blood pressure in a human. The method comprises calculation of blood flow from aortic pressure and integration of the flow over the systolic period, the aorta being regarded as a transmission line supplemented with a windkessel compliance and the pressure/volume relationship in the aorta as an arctangent relationship. The characteristic of the transmission line and the windkessel compliance are continuously adapted to the pressure of the windkessel compliance for each stroke concerned. Additionally, to calculate the aortic flow to the windkessel, the flow through the peripheral resistance is added to the flow into the windkessel compliance.
According to Langewouters et al., J Biomechanics (1984) 17, 425-435, the relationship between the thoracic cross sectional area (CSA) of the aorta and the pressure in the aorta can be specified using the formula:
                              C          ⁢                                          ⁢          S          ⁢                                          ⁢                      A            ⁡                          (              Pa              )                                      =                  C          ⁢                                          ⁢          S          ⁢                                          ⁢          A          ⁢                                          ⁢                      max            [                          0.5              +                                                1                  π                                ⁢                                  arctan                  ⁡                                      (                                                                  Pa                        -                                                  P                          0                                                                                            P                        1                                                              )                                                                        ]                                              (        1        )            where CSA(Pa) is the thoracic cross sectional area as a function of the pressure Pa, CSAmax is the limit cross sectional area at very high pressure, and P0 and P1 are, respectively, the pressure at the point of inflection of the relationship and the slope of the relationship at the point of inflection. The slope is defined by the width between the points at one-half and three-quarter of CSAmax. The values of the parameters CSAmax, P0 and P1, are known from the work of Langewouters et al. In particular, these parameters depend on the sex and age of the person. In the prior art these parameters are used in monitoring applications, wherein the pulse contour model is used. However, the value for CSAmax has a dispersion of approximately 20%. Therefore, it is not possible to determine the values of the components in the pulse contour model in an absolute sense without more exact calibration, as a standard deviation of approximately 20% is regarded as unacceptable, in particular in patient monitoring applications.
U.S. Pat. No. 6,348,038 describes another method for measuring cardiac output using pulse contour analysis. A non-linear transformation is used to correct for the changing characteristics of the arterial system with pressure and autocorrelation is then used to derive the cardiac output. The method determines the nominal stroke volume from the modulus of the first harmonic from the arterial pressure and obtains nominal cardiac output and systemic vascular resistance. In this method, the pressure waveform obtained from a patient is transformed into a volume waveform, for instance via a ‘look-up’ table, with the mean of the data of the pressure-volume relationship of the arterial system based on a population average. The basic approximation to a look up table is known in the art.
A series of pressure-volume curves is described in Remington et al., “Volume elasticity characteristics of the human aorta and prediction of the stroke volume from the pressure pulse”, Am. J. Physiol (1948) 153: 298-308. A used equation has the form of:
                              SV          N                =                              P                          1              ⁢              H                                            H            ⁢                                                  ⁢            R            ⁢                                                  ⁢                          exp              ⁡                              (                                                      -                    0.0092                                    ⁢                                                                          ⁢                  M                  ⁢                                                                          ⁢                  A                  ⁢                                                                          ⁢                  P                                )                                                                        (        2        )            where SVN is nominal Stoke Volume, P1H is modulus of first harmonics of the arterial pressure, MAP is mean arterial blood pressure and HR is heart rate. The value 0.0092 is related to a population average.
Also, Kortnet et al. J. Physiol. (1998) 512: 917-926 describe the non-linear relationship between the cross sectional area and pressure of the aorta using the formula:
                              C          ⁢                                          ⁢          S          ⁢                                          ⁢                      A            ⁡                          (              Pa              )                                      =                              A            ⁢                                                  ⁢            min                    +                                                                      A                  ⁢                                                                          ⁢                  max                                -                                  A                  ⁢                                                                          ⁢                  min                                            ⁢                                                                                  1              +                              exp                ⁡                                  [                                                            (                                                                        P                          0                                                -                        Pa                                            )                                        /                                          P                      1                                                        ]                                                                                        (        3        )            where, CSA(Pa) is the thoracic cross sectional area of the aorta as a function of the pressure Pa, Amin is the area at zero pressure (usually Amin=0), Amax is the area at very high pressure, P0 is the inflection point, and P1 is the slope of the relationship at the point of inflection. The values for the parameters of this relationship are known based on population averages.
Accurate cross sectional area (CSA) measurements of the aorta are known from Boulnois et al J Clin Monit (2000) 16:127-140. The described non-invasive transesophageal ultra-sound approach allows a continuous measurement over time of the CSA of the aorta, resulting in a patient-specific CSA determination.
It is now accepted in the art that all existing pulse contour methods require calibration for improved accuracy because the value of the parameters of the used pressure/volume or pressure/CSA relationships are based on population averaged values and are, thus, not necessarily valid for a specific individual patient.
A calibration, for example based on the thermodilution or indicator dilution method, as described in EP-A-1 154 720 is highly repeatable and only one single determination is required to give mean cardiac output. However, these thermodilution calibration methods require highly invasive techniques of placement of catheters into the blood stream of a patient.